It is known that antenna performance is dependent on the size, shape, and material composition of constituent antenna elements, as well as the relationship between the wavelength of the received/transmitted signal and certain antenna physical parameters (that is, length for a linear antenna and diameter for a loop antenna). These relationships and physical parameters determine several antenna performance characteristics, including: input impedance, gain, directivity, signal polarization, radiation resistance and radiation pattern.
Generally, an operable antenna should have a minimum physical antenna dimension on the order of a half wavelength (or a quarter wavelength above a ground plane) (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted energy. A quarter wavelength antenna (or multiple thereof) operative above a ground plane, exhibit properties similar to a half wavelength antenna. Generally, communications product designers prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched to the transmitting and receiving components of the communications system, and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns).
Certain antennas, such as a meanderline antenna described below, present an electrical dimension that is not equivalent to a physical dimension of the antenna. Thus, such antennas should exhibit an electrical dimension that is a half wavelength (or a quarter wavelength above a ground plane) or a multiple thereof
Quarter wavelength antennas operable in conjunction with a ground plane are commonly used as they present smaller physical dimensions than a half wavelength antenna at the antenna resonant frequency. But, as the resonant frequency of the signal to be received or transmitted decreases, the antenna dimensions proportionally increase. The resulting larger antenna, even at a quarter wavelength, may not be suitable for use with certain communications devices, especially portable and personal communications devices intended to be carried by a user.
A meanderline-loaded antenna (MLA) represents a slow wave antenna structure where the physical dimensions are not equal to the effective electrical dimensions. Such an antenna de-couples the conventional relationship between the antenna physical length and resonant frequency, permitting use of such antennas in applications where space for a conventional antenna is not available. Generally, a slow-wave structure is defined as one in which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(∈r)sqrt(μr))=λf. Since the frequency remains unchanged during propagation through a slow wave structure, if the wave travels slower than the speed of light (i.e., the phase velocity is lower), the wavelength within the structure is lower than the free space wavelength. Thus, for example, a half wavelength slow wave structure is shorter than a half wavelength structure where the wave propagates at the speed of light (c). Slow wave structures can be used as antenna elements (i.e., feeds) or as antenna radiating structures.
Since the phase velocity of a wave propagating in a slow-wave structure is less than the free space velocity of light, the effective electrical length of these structures is greater than the effective electrical length of a structure propagating a wave at the speed of light. The resulting resonant frequency for the slow-wave structure is correspondingly increased. Thus if two structures are to operate at the same resonant frequency, as a half-wave dipole for instance, the structure propagating the slow wave is physically smaller than the structure propagating the wave at the speed of light.
Slow wave structures are discussed by A. F. Harvey in his paper entitled Periodic and Guiding Structures at Microwave Frequencies, in the IRE Transactions on Microwave Theory and Techniques, January 1960, pp. 30–61 and in the book entitled Electromagnetic Slow Ware Systems by R. M. Bevensee published by John Wiley and Sons, copyright 1964. Both of these references are incorporated by reference herein.
A typical meanderline-loaded antenna (also known as a variable impedance transmission line (VITL) antenna) is disclosed in U.S. Pat. No. 5,790,080. The antenna comprises two vertical conductors and a horizontal conductor, with a gap separating each vertical conductor from the horizontal conductor.
The antenna further comprises one or more meanderline variable impedance transmission lines electrically bridging the gap between the horizontal conductor and each vertical conductor. Each meanderline coupler is a slow wave transmission line structure carrying a traveling wave at a velocity less than the free space velocity. Thus the effective electrical length of the slow wave structure is considerably greater than it's actual physical length. The relationship between the physical length and the electrical length is given byle =∈eff×lpwhere le is the effective electrical length, lp is the actual physical length, and ∈eff is the dielectric constant (∈r) of the dielectric material containing the transmission line. By using meanderline structures, smaller antenna elements can be employed to form an antenna having, for example, quarter-wavelength properties.
A schematic representation of a prior art meanderline-loaded antenna 10, is shown in a perspective view in FIG. 1. This embodiment of a meanderline-loaded antenna 10 comprises two spaced-apart vertical conductors 12, a horizontal conductor 14 spanning the distance between the two vertical conductors 12, and a ground plane 16. The vertical conductors 12 are physically separated from the horizontal conductor 14 by gaps 18, but are electrically connected to the horizontal conductor 14 by two meanderline couplers, (not shown), one meanderline coupler for each of the gaps 18, to thereby form an antenna structure capable of radiating and receiving RF (radio frequency) energy.
The meanderline couplers (also referred to as slow wave structures) electrically bridge the gaps 18 and, in one embodiment, have controllably adjustable lengths for changing the performance characteristics of the meanderline-loaded antenna 10. In one embodiment of a meanderline coupler, segments of the meanderline transmission line can be switched in or out of the circuit with negligible loss, to change the effective electrical length of the meanderline coupler, thereby changing the effective antenna length and thus the antenna performance characteristics. The switching devices can be located in high impedance sections of the meanderline transmission line, to minimize current through the switching devices, limiting dissipation losses and maintaining the antenna efficiency.
Like all antennas, the operational parameters of the meanderline-loaded antenna 10 are affected by the wavelength of the input signal (i.e., the signal to be transmitted by the antenna) relative to the antenna effective electrical length (i.e., the sum of the meanderline coupler lengths plus the antenna element lengths). According to the antenna reciprocity theorem, the antenna operational parameters are also equally affected by the received signal frequency. Two of the various modes in which the antenna can operate are discussed below.
FIG. 2 shows a perspective view of a meanderline coupler 20 constructed for use with the meanderline-loaded antenna 10 of FIG. 1. Two meanderline couplers 20 are generally used with the meanderline-loaded antenna 10; one meanderline coupler 20 bridging each of the gaps 18 illustrated in FIG. 1. However, it is not necessary for the two meanderline couplers to have the same physical (or electrical) length.
The meanderline coupler 20 of FIG. 2 is a slow wave meanderline element (or variable impedance transmission line) constructed in the form of a folded transmission line 22 mounted on a dielectric substrate 24, which is in turn mounted on a plate 25. In one embodiment, the transmission line 22 is constructed from microstrip transmission line elements. Sections 26 are mounted close to the substrate 24; sections 27 are spaced apart from the substrate 24. In one embodiment, as shown, the sections 28 connecting the sections 26 and 27 are mounted orthogonal to the substrate 24. The distance between the alternating sections 26 and 27 and the substrate 24 gives the sections 26 and 27 different impedance.
As shown in FIG. 2, each of the sections 27 is approximately the same distance above the substrate 24. However, those skilled in the art recognize that this is not a requirement for the meanderline coupler 20. Instead, the various sections 27 can be located at different distances above the substrate 24. Such modifications change the electrical characteristics of the coupler 20 from the embodiment employing uniform distances. As a result, the characteristics of the antenna employing the coupler 20 are also changed. The impedance (and thus the effective electrical length) presented by the meanderline coupler 20 can also be changed by changing the material or thickness of the microstrip substrate or by changing the width of the sections 26, 27 or 28. In any case, the meanderline coupler 20 must present a controlled (but controllably variable if the embodiment so requires) impedance.
The sections 26 are relatively close to the substrate 24 (and thus the plate 25) to create a lower characteristic impedance. The sections 27 are a controlled distance from the substrate 24, wherein the distance determines the characteristic impedance and frequency characteristics of the section 27 in conjunction with the other physical characteristics of the folded transmission line 22.
The meanderline coupler 20 includes terminals 40 and 42 for connection to the elements of the meanderline-loaded antenna 10. Specifically, FIG. 3 illustrates two meanderline couplers 20, one affixed to each of the vertical conductors 12 such that the vertical conductor 12 serves as the plate 25 from FIG. 2, forming a meanderline-loaded antenna 50. One of the terminals shown in FIG. 2, for instance the terminal 40, is connected to the horizontal conductor 14 and the terminal 42 is connected to the vertical conductor 12. The second of the two meanderline couplers 20 illustrated in FIG. 3 is configured in a similar manner.
The operating mode of the meanderline-loaded antenna 50 (see FIG. 3) depends upon the relationship between the operating frequency and the effective electrical length of the antenna, including the meanderline couplers 20. Thus the meanderline-loaded antenna 50, like all antennas, exhibits operational characteristics as determined by the ratio between the effective electrical length and the transmit signal frequency in the transmitting mode or the received frequency in the receiving mode. Different operating frequencies will excite the antenna so that it exhibits different operational characteristics, including different antenna radiation patterns. For example, a long wire antenna may exhibit the characteristics of a quarter wavelength monopole at a first frequency and exhibit the characteristics of a full-wavelength dipole at a frequency of twice the first frequency.
FIGS. 4 and 5 depict the current distribution (FIG. 4) and the antenna electric field radiation pattern (FIG. 5) for the meanderline-loaded antenna 50 operating in a monopole or half wavelength mode as driven by an input signal source 44. That is, in this mode, at a frequency of between approximately 800 and 900 MHz, the effective electrical length of the meanderline couplers 20, the horizontal conductor 14 and the vertical conductors 12 is chosen such that the horizontal conductor 14 has a current null near the center and current maxima at each edge. As a result, a substantial amount of radiation is emitted from the vertical conductors 12, and little radiation is emitted from the horizontal conductor 14. The resulting field pattern has the familiar omnidirectional donut shape as shown in FIG. 5.
A second exemplary operational mode for the meanderline-loaded antenna 50 is illustrated in FIGS. 6 and 7. This mode is the so-called loop mode, operative when the ground plane 16 is electrically large compared to the effective length of the antenna. In this mode the current maximum occurs approximately at the center of the horizontal conductor 14 (see FIG. 6) resulting in an electric field radiation pattern as illustrated in FIG. 7. The antenna characteristics displayed in FIGS. 6 and 7 are based on an antenna of the same effective electrical length (including the length of the meanderline couplers 20) as the antenna depicted in FIGS. 4 and 5. Thus, at a frequency of approximately 800 to 900 MHz, the antenna displays the characteristics of FIGS. 4 and 5, and for a signal frequency of approximately 1.5 GHz, the same antenna displays the characteristics of FIGS. 6 and 7. By changing the antenna element electrical lengths, monopole and loop characteristics can be attained at other frequencies.
Generally, the meanderline loaded antenna exhibits monopole-like characteristics at a first frequency and loop-like characteristics at a second frequency where there is a loose relationship between the two frequencies, however, the relationship is not necessarily a harmonic relationship. A meanderline-loaded antenna constructed according to FIG. 1 and as further described herein below, exhibits both monopole and loop mode characteristics, while typically most prior art antennae operate in only a loop mode or in monopole mode. That is, if the antenna is in the form of a loop, then it exhibits a loop pattern only. If the antenna has a monopole geometry, then only a monopole pattern can be produced. In contrast, a meanderline-loaded antenna according to the teachings of the present invention exhibits both monopole and loop characteristics.
One important antenna operational parameter is the antenna input impedance, comprising resistive and reactive components that are presented at the antenna input terminals. The resistive component results from antenna radiation and ohmic losses. The reactive component stores energy within the antenna. It is desirable for the resistive component to be constant at the antenna resonant frequency and to have a moderate value, e.g., 50 ohms, at this frequency. The magnitude of the reactive component should be small, ideally zero, to limit the energy stored in the antenna. For an antenna operative over a band of frequencies or at several disparate frequencies, it is desired that the input impedance be about 50 ohms over the frequency range of interest and for the reactive component to be minimal over this same range. The 50 ohm value is conventional in the art, as explained below.
Connecting an antenna to other communications components presents several physical and electrical interface challenges, whether the antenna is operative with spatially proximate communications components such as in a portable communications device, or physically distant from these components such as when mounted on an antenna mast above the earth's surface. For effective operation of the antenna and the communications device, these challenges must be resolved.
In any antenna installation, when operating in a receiving mode, an antenna 70 is typically connected to a filter 72 by a transmission line 73. See FIG. 8. The received signal is filtered to remove unwanted frequency signals received by the antenna 70. Since the received signal is relatively weak, the filtered signal is amplified in an amplifier 74 prior to processing through other components that extract information carried by the received signal.
In the transmitting mode, the antenna 70 is connected to a power amplifier 78 (via a transmission line 79) for boosting the signal strength prior to radiation from the antenna 70. See FIG. 9.
As mentioned above, to minimize electrical losses, it is known to match an output impedance of the filter 72 to an input impedance of a the transmission line 73 (typically 50 ohms), and to match an output impedance of the transmission line 73 (again 50 ohms) to an antenna input impedance. The matching is accomplished by one or both of a matching network 80 associated with the filter 72 and a matching network 82 associated with the antenna 70. Although exact impedance matching of such components is academically desired, pragmatically it is known that two components can be considered to be matched if the impedance values are within a range of about 25% to 50% of either impedance value.
A filter, such as the filter 72, often possesses a negative or capacitive reactance at its output terminals, whereas an antenna (for instance, a loop antenna) may present an inductive or positive reactance at its input terminals. When the filter 72 and the antenna 70 are physically separated and connected with the transmission line 73, as in FIG. 8, the antenna positive input reactance must be matched, using the matching components 82, to a 50 ohm real load presented by the transmission line 73. This is accomplished by configuring the matching components 82 to present a conjugate impedance relative to the antenna impedance. Such a match provides maximum power transfer and efficiency between the antenna 70 and the transmission line 72.
Likewise, the filter 72 requires the matching components 80 to present a conjugate match to the transmission line 73, while transforming the real part of the impedance to 50 ohms to match the transmission line impedance. Effecting these two impedance matching requirements permits maximally efficient operation of the filter 72 and antenna 70 with the intervening transmission line 73. The matching components 80 and 82 can be connected at any point or break in the transmission line 73. Unfortunately, the added matching components add cost and additional power loss, resulting in unrecoverable signal losses to heat in the matching components.
Similarly, a power amplifier output impedance is matched to the antenna input impedance through a matching network 84 or the matching network 82. Certain power amplifiers (also referred to as RF (radio frequency) amplifiers since they operate on RF signals) are comprised of a differential output transistor pair. Thus the output signal from these amplifiers must be converted to a single ended drive to interface with the 50-ohm transmission line 79, which in turn connects to the antenna 70. A balun is a device that can be used to convert from a differential output to a single-ended output.
In the industry there is an historical reliance on a 50-ohm impedance match between the antenna and other front end components (e.g., filter, amplifier). The historical importance of the 50-ohm impedance match is predicated on the impedance characteristics of certain transmission lines comprising dielectric materials and two electrical conductors arranged in coaxial geometry. The transmission lines are designed to minimize losses over long distances. For this geometry, the optimal transmission line impedance is calculated to be in the range of 50 to 75 ohms. Thus this value has defined the 50 ohm impedance matching between the antenna and other font end components when connected by a transmission line.
Since small portable devices rely on very short transmission lines due to the proximity of the antenna and the front end components, there is no need to require the standard impedance of 50 ohms between these elements. There are also advantages to be gained, i.e., minimizing losses, by avoiding the impedance transformation from the amplifier output stage to 50 ohms and then reconversion from 50 ohms to the antenna impedance at resonance, which is often less than 50 ohms. It is therefore advisable to connect the antenna to the amplifier or the filter through an impedance matching element of other than 50 ohms.
In addition to the electrical impedance matching, physical interface issues are important whenever an antenna is installed proximate other components of the communications device. It is necessary to properly interface the device elements to limit deleterious component interactions. The transmission line connecting the components must be properly routed, and there are also component shielding issues to consider. These design concerns add cost and complexity to the design process, and also to the cost of debugging the device to resolve problems caused by unexpected component interactions.
The same issues of physical and electrical interfacing are present in radio frequency transmitting and receiving installations utilizing a mast-based antenna connected via a transmission line to ground-based receiving and transmitting components typically housed in a shelter, enclosure or cabinet at the base of the antenna mast or tower. Such installations are used for long distance communications. Antennas for several different wireless services or antennas operating at different frequencies for the same wireless service, frequently share the antenna mast. With the proliferation of wireless devices and the base station antennas to service them, and the attendant crowding of the RF spectrum, co-interference caused by spatially close wireless service antennas operating at adjacent or nearby spectral frequencies is an increasingly serious problem.
At mast sites, or any site where radio services are co-located, the conventional technique for reducing interference is through the use of in-line filters providing any of the known filter functions, such as low pass, high pass, bandpass, band reject, notch, diplex or duplex. These filters are generally purchased from suppliers other than the antenna supplier and thus must be mechanically fitted to and electrically matched (i.e., impedance matched) to the transmission line characteristics and to the antenna. The filters are typically co-located with the receiver/transmitter equipment or disposed in-line, that is, within the transmission line. The filter can be tunable under control of the receiver/transmitter such that as the receiver or transmitter is tuned, the appropriate frequency components are passed or blocked by the filter. Whether located in-line or with the receiver/transmitter, additional space is required to accommodate the filters. In the latter situation, space must be made available at the base of the tower, where it is at a premium. In-line filters require special cables and connectors to connect the filter into the transmission line. These connectors can become a source of interfering radiation for other nearby transmitting and receiving devices. Signal leakage is especially prevalent at the cable connectors and increases as the cable deteriorates due to water intrusion and other weathering effects.
To further reduce interference, high isolation transmission lines are employed between the antenna at the top of the mast and the receiving/transmitting equipment at ground level. The transmission lines, which are by necessity expensive and bulky to achieve the required high-isolation properties, are designed to prevent the unintended reception of interfering signals from nearby transmitting antennas and nearby leaking transmission lines. The high-isolation lines are also designed to limit the outgoing RF′ leakage that may cause problem for adjacent transmission lines and receiving/transmitting equipment.
The transmission lines themselves are also problematic as water leakage, physical damage (e.g. gouging or denting of the cable) or loose connectors between line segments can change the transmission line impedance and thereby affect the line's performance. At an exemplary antenna tower, it is determined that the transmission line between the tower and the receiver/transmitter is particularly susceptible to interference from another antenna mounted on the tower and operating at a frequency f. To remedy this situation, a notch filter is installed in the transmission line. The installation requires opening the high-isolation transmission line and installing the notch filter to attenuate the troublesome signal. High isolation connectors are required for this installation, and upon completion, the system performance must be tested, as it is known that the installation of filters may disrupt and modify the transmission line characteristics and thus the performance of the entire system.
Antennas employed in these wireless applications as mounted on towers and masts include any of the well known antenna types: half-wave dipoles, loops, horns, patches, parabolic dishes, etc. The antenna selected for any given application is dependent on the requirements of the system, as each antenna offers different operational characteristics, including: radiation pattern, efficiency, polarization, input impedance, radiation resistance, gain, directivity, etc. A meanderline-loaded antenna can also be used in these installations.